Thermionic valve circuits



Patented Sept. '1 1942 UNITED STATES PATENT OFFICE 'rimnmomc VALVECIRCUITS Bertram Morton Hadfleld, Harrow Weald, England, assignor toAssociated Electric Laboratories Inc., Chicago, 111., a corporation ofDelaware Application July 10, 1940, Serial No. 344,642 In Great BritainAugust 5, 1939 scams. (curs-171) The present invention relates tothermionic valve circuits and is more particularly concerned withthermionic valve circuitsin which a substantially constant alternatingoutput may be which produces substantially zero anode cur rent. At anygiven value of control grid potential, the slope of the tangent to thecharacteristic will represent the mean change of anode current per unitchange of control grid potential, so that if an additional alternatinginput be applied to the control grid. the output of alternating anodecurrent will be proportional to the product of the input alternatingcontrol grid voltage and the slope of the tangent, the latter beingexpressed as a ratio of the change in anode current per unit change incontrol grid potential.

It can be shown that the slope of the tangent,

as defined above is proportional to the controlgrid potential (reckonedfrom the zero anode current potential) and as the latter is proportionalto the square root of the anode current, it follows that if theisquareroots of the anode currents at various control grid potentials areplotted, a straight line will be obtained which represents the variationof the slope of the tangent at these control grid potentials.

between the alternating input and the outputwill be a parabola, whosemaximum value occurs at a control grid potential of one half thedifference between the initial bias (with no input) and the biasnecessary to produce zero anode current. The output will be zero-whenthe alternating input is zero and also when the bias due to thealternating input is such as to produce zero slope (i. e. zero anodecurrent).

Figures 1 and 2 show characteristics explaining the operation of theinvention.

According to one feature of the invention a direct current bias, whosevalue is proportional to the alternating input, is applied to a controlgrid of a thermionic valve in series with the alternating input, theinitial bias on the control grid being very small so that a large anodeourrent flows in the absence of an alternatingim put.

put with inputcan be determined by taking the product of the controlgrid directv current potential (due to the alte'matinginput) and theslope of the control grid/anode current char- As describedabove, theoutput of alter nating anode current will be proportional to theacteristic at each potential. Since the relation between these twovariables is linear, the relation Figure 3.and 4 showspeciiicembodiments of V the invention.

Thus referring to Fig. 1 of the accompanying drawing, the mutualconductance or slope of the grid-volts/anode current characteristic hasbeen plotted on a verticalaxis as a, whilst the corresponding gridvoltage Va is plotted on a horizontal axis. As pointed out above therelation between 9 and Va is a straight line such as AB, if the gridvolts/anode current characteristic is a parabola. -'I'he parabola APOrepresents the relation between the output and input voltages.

Let the difierence in biasses on the control grid occurring at an inputbias 01 W2. The change,

of output above this value is small depending on the working limitsassigned to V, For instance, assuming that V varies from W3 to 2W3, thenvG will varysfrom 2G/3 to G/3 respectively, .and

therefore the output at these values is '2VG/9. The change in outputtherefore, over this range of input levels (6 db. on voltage ratiobasis) is from VG/4 to 2VG/9, or a change 'of- 11.2%.

As' a further feature of the invention it is proposed to reduce the rateof change of direct current bias on the control grid due to thealterhating input, at a predetermined value of the bias, leavingthealternating .input to the grid unaltered. For thepurpose ofiliustratingthe effect of this alteration, the initial working range taken abovewill be considered, i. e. a working range of 6 db. over a change of- Vfrom V/3 to At a bias of 2V/3 on the control grid due to the alternatinginput 1. e. at the point x, let

the rate of change of such bias with increasing input be reduced to onequarter. It calf be (1+p), where n is the number or times the rate shownthat this amounts to altering the slope of changeol grid biasis altered.

of the linear relationshp between V and at this Urn) point to onequarter of that formerly obtaining m i. e. changing AB to CD. Theintercepts of this 6 working range= -(l p), from (3) new linearrelationship onthe V and G axes (V I can be shown to be 2V and 'G/2respectively. =n(1+p) (7) Hence with increasing input theoutput willdescribe a fr sh parabola whose maximumwm It can be shown that values ofp greater than 1 occur et-en in corresponding to v and at an 10 are notprofitable, besides adding to the comanode current/ control slope of the0f the bias circuit Likewise Values value f the maximum being th refore4. of 11 greater than 3 do not add materially to the This ts the samemaximum value as was tt range and affect the change of output to ,agreater previous t e redu ti n-of the t of increase degree. If n istaken at 3 the calculated range is of bias, whilst at an inputcorresponding to 15 16 I 4V/3 the output will against 2VG/9 Hence t Theprocess of reduction of rate of change of resultant t t curve 1 he CQPOand bias with increasing input at such predetermined working range hasbeen extended by a further 4, points can; of course, be extended almostinde- 6 db. with the same change in putput of 11.2% fini l each r uctioniving an extension or as was, formerly obtained. 20 working range ofamount equal to the initial h erder estimate the change of outputvoltrange (the range being expressed in terms of ratio age f r a givenrange f mputhlevelsi with one i. e. deci-bels for instance). Thedetermination change of bias ratio, the general case will now he of thepoints at which reduction of rate of change taken I e is desired dependson the amount of change toler- InFig. 2 the same basic characteristic isshown, able in the 013911?- ovel the Wmking but the rate of change ofgrid bias is altered, at. Furtherymra by using two W m push-pun a giveninput by so that the intercepts f. and applym the direct current b asderived from the new slope/grid bias line are Glu and 12V re.- theinputs to the control grids m Parallel" the speet1ve1y Let t f rm of tequation for t changes in the direct current component of the slope/gridbias line be g=A+C'Vg, then for t anode currents are prevented fromaflecting the original line, waveform of the alternating output. Inaddition,

- a as is well known,the output impedance,such as g=G+- -Vg (1) atransformer, need not be designed with a view to carrying large directcurrents.

and for the expanded line. In order that the application of theinvention G G Va may be more easily understood, reference should g=; (2)be made to Figs. 3 and 4 of which Fig. 3 shows a specific method forobtaining the desired reduc- Equatins ese wi eiv t e p i o intersectiontion in rate,of change of control grid bias at G g 40 a specific biaswhile Fig. 4 shows acomplete cir- +T" 9 ;+,;T 9 cult. The inputtransformer, which may also supply the alternating input to the controlgrid ,,Vg= or grids, is arranged with a winding, which by "(l-n) 4 meansof any well known rectifyingsystem supe 1v (3) plies a direct currentbias between the cathode 1+1 and control grids. For a reduction in rate01' Th change of say, one quarter, the bias is developed 8 van}? of atthe pant 9f intersection across the following network shown in Fig. 3.

Between the positive and negative output termi- V 1 5o nals of'therectifier system is connected in series G 4 aresistance R, a battery E(or other source of TT-Fr' direct current potential) in such a manner asto tend to oppose currents from the rectifier vsysgg g output at thepoint of tem, a rectifier MR in such a manner as to tend e v I to allowcurrentsto pass from therectifler sys- Q ,L L (5) tem, and a resistancer. The cathodes of the -i- 0 f" I valves are connected to the junctionof R and E. But th maximum t t is 4 V whilst the grids are connected(via the push-pull T erefore the ratio ofminimum to maximum out- 2:?51vintage) the negative put is 4n 11 the backwardresistance 01' therectifier be .very large compared with R, then substantially +n) f 1 1no potential exists between the cathodes and grids It theworking rangeis expressed as the ratio in the absence of an alternating input. Whenof the grill bias at the point of intersection to an alternating inputis applied, until the voltage that which'initially produces the sameoutput, delivered by e r fi sy m exceeds no then from inspection it isplain that if the former c r en will pass throu h the rectifier MR an byV1 then the latter 'will be V-Vl. Also the the whole of the bias isapplied to the control total range will be the "initial" range (from V1grid of the valve When the rectifier voltage to V-Vl) multiplied by thenumber of times the exceeds E a current passes through the rectifier t ichange grid bi is lt d pl s n MR. in its forward dllECtiOll, 50 that itbecomes Therefore the working rangeis of small resistance compared to1'. Therefore (V1) for increased input the rate of change of biasvoltageiis reduced to r/R+r, after a voltage E is attained-and if areduction ratio or one cuarter.

' sources, such as the 50 cycle mains supply. This anacea is desired 1'will be made equal to one quarter of thesumofRandr. I i

For a range of levels of. some 20 db. it is generally not necessary toapply more than one "reduction in ratio as most valve characteristics 5'tend to deviate from the true parabola at low anode currents, theeffect being to prolong the characteristic over a further small range ofcontrol grid potential. This factor gives .an extra working range overand above that formerly l0 calculated. Likewise the points of minimumout- 'put indicated by the calculations (1. e. where the rate of changeof bias is altered) are not so pronounced in. practice, because of thegradual change of; the resistance .of the rectifier MR from a highbackward value to a low forward value. 'This factor may be made use ofwitlr advantage to produce a more nearly constant 7 output over thedesired range.

Referring now to Fig. 4,-in which an input transformer Tl supplies boththe grid bias and the alternating voltages to push-pull pentodevalves Viand V2'from' its secondary winding. Apush-pull rectifier system P and Qis shown supplying unidirectional voltage to the grid bias 5 circuit.The output impedance-R0 may be made Owing to the balanced nature of theoutput circuit, it is apparent that the device is immune from surges onthe supply. i 4

As the portion of the bias network applied-between the cathodes andgrids is of relatively high impedance in the absence of an alternatinginput it is found in practice that voltages may be 4 picked up on thisimpedance from surrounding results in a corresponding modulation, of theoutput and can be eliminated by placing a suitable .condenser Cl acrossthe impedance. Such 5 a device however,.introduces1a relatively longcharging time constant for input voltages producing a bias :gvhich issumcient to cause .the network to assume its reduced ratiocondition 7(since the condenser CBI is now' effectively 5; charged via R anddischarges via r). .In orden ,to obviate this effect it is proposed toconnect a further condenser C2 across R: such that the time constants02R and "Cl: are equal. The

condenser for the rectifier system which produces the bias.

In order that full advantage may be taken of the parabolic nature of.the anode current/grid voltage characteristic of a thermionic valve(since a more rapid attainment of constancy of output with low levels isthereby achieved) it is desirable that the static characteristicnormally obtained by neglecting-the eflects of an anode load impedance,shallbe used. For this reason it is preferred to use pentode valves, .inwhich the anode loa has relatively little effect on the static characte'tic.

It will .be realised that if the output is to be kept reasonablyundistorted at input le els ex-, ceedingvthe upper limit of the workinrange, so as to obtainthe full benefit of the reduction in output atthis point, the alternating input to the grids must be kept low enoughto avoid anode bend rectification, It has been found that if Q I A theratio of the bias voltage produced by the rectifier systemto .thealternating peak input per 'grid is some ten to one thebutput remainssubstantially pure. I

4 Despite the reduction in output which this entailsas compaied with theoutput of the valve as a linear amplifier for the same anode impedan'ce,it is possible to attain a substantial percentage .of the maximum outputby the use of pentode valves in conjunction with high anode loadimpedances. As the effective alternating output current over the workingrange will be constant and smaller than that giving themaxincrease theworking anode impedance (over and above that normally used), to a valueapproximating to the dynamic impedance of the pentode valve, withoutintroducing distortion due to curvature of the anode or gridcharacteristics, and more especially when a, push-pull system is used.

.In the foregoing description of the principles and embodiments of theinvention, where reference is made to the control grid, this electrodeneed not necessarily be that nearest the cathode or filament, althoughthis is generally advantageous since the maximum sensitivity is hereobtained. The invention can be applied to control on the suppressor gridof a pentode valve or valves in push-pull, whose anodecurrent/suppressor grid characteristic is closely parabolic.

In using the suppressor grid, advantage can be taken 'of the fact thatthe gain of the.valve as a whole tends to remain constant withvariations in'suppressor potential (since the screen current andconductance are increased as those of the anode are diminished), byderiving the bias proportional to the alternating input from the commonelectron path (for instance from a cathode'impedance). Likewise othertypes of valves having oneor more control grids with a suitableparabolic characteristic can be us such as the heptode and octode.

The biasproportional to the input need not be derived directly therefromand more especially when the resulting load onthe input is notpermissible; it can be derived from a thermionic amplifier or from a'self rectifying valve of the cathode follower type.

One'of the applications of the invention occurs imum output from thevalve, it is possible to I in the-reception of signals of alternatingcurrent 2 over telephonic lines, where the invention forms the firststage of a receiver connected to the line.

- Such signals may be received. at widely differing condensers may thenform the normal storage levels and may be of. definite frequency orfrequencies, whilst it is desired to prevent the response of thereceiver to currents which may rents becomes higher.

I claim; i I 1. An amplifier having a space dischargedevice and an inputtransformer, a rectifier circuit ineluding a portion of the secondary ofsaid trans-:

former for initially applying-a grld 2. An amplifier as claimed inclain'i 1 in which.

said network is nonconductive until the grid potential reaches a certainpoint and then becomes conductive to act as a shunt for a part of saidrectifier circuit.

3. In a thermionic amplifier, an input transi'ormer, a space dischargedevice, a connection from said transformer to the grid of said devicefor supplying alternating current thereto for emplification, a rectifiercircuit also connected to said transformer and to said grid to supplybiasing potential thereto derived from the input, a second rectifiercircuit connected to said first rectifier circuit, a source of potentialand a rectiher in said second circuit normallyeflective to prevent anyflow of current therein, said second circuit eilective when the currentin said first circuit reaches a certain value to cause a current to fiowtherein from the first circuit to thereby effect a change in the ratioof thebiasing potential to the input volume.

4. In a thermionic amplifier, a pair of space discharge devicesconnected in push' pull rela-- tion, an input transformer therefor, theouter ends of the secondaryofsaid transformer connected through a pairof rectifiers to the filamentsof said devicesia connection from eachcontrol grid of said devices to a point on said secondary, a secondconnection from the filaments of said devices through a source ofcurrent and a rectifier to the center point of said secondary, saidpairof rectifiers supplying a grid potential to said control gridswhenever al- 4 ternating current is supplied to the primary of saidtransformer, said last connection being effective, after the input-volume reaches a,cer-

tain point, to vary the rate of change in the grid

